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11.8.3 Addition and condensation polymers

Repeat units and linkages

Definition

Repeat unit

The smallest section of a polymer chain that is repeated over and over to build up the whole molecule.

  1. Every polymer chain is built from one short section that appears again and again, called the repeat unit.
  2. When you draw a polymer, you draw just the repeat unit inside square brackets.
    1. A bond sticks out from each end of the repeat unit and passes through the brackets, showing where it joins the next unit.
    2. A subscript nnn after the closing bracket shows that the unit is repeated a large number of times.
  3. The bond or group of atoms that joins one repeat unit to the next is called the linkage.
  4. The type of linkage tells you how the polymer was made.
    1. In an addition polymer, the units are joined by carbon-carbon single bonds, C-C\text{C-C}C-C.
    2. In a polyamide, the units are joined by amide linkages, -CONH-\text{-CONH-}-CONH-.
    3. In a polyester, the units are joined by ester linkages, -COO-\text{-COO-}-COO-.

Addition polymers from alkenes

Definition

Addition polymerisation

A reaction in which many unsaturated monomers join together, by opening their carbon-carbon double bonds, to form a polymer as the only product.

  1. Any alkene can act as a monomer for addition polymerisation, because every alkene has a C=C\text{C=C}C=C double bond that can open.
  2. To deduce the repeat unit from a given alkene, follow the same steps every time.
    1. Draw the alkene with the two carbon atoms of the C=C\text{C=C}C=C bond side by side and every other group above or below them.
    2. Change the C=C\text{C=C}C=C double bond into a C-C\text{C-C}C-C single bond.
    3. Keep every other atom or group, such as H\text{H}H, CH3\text{CH}_3CH3​ or Cl\text{Cl}Cl, attached to the same carbon as before.
    4. Add a bond out of each side, then draw the brackets and nnn.
  3. Only the two carbon atoms of the double bond become part of the main chain, so the repeat unit of an addition polymer always has a two-carbon backbone.
    1. Any other carbon atoms, such as the CH3\text{CH}_3CH3​ group in propene, form side groups that hang off the chain.
  4. The repeat unit contains exactly the same atoms as the monomer, because addition polymerisation forms no other product.
  5. To deduce the monomer from an addition polymer, reverse the steps.
    1. Pick out the two backbone carbon atoms of one repeat unit, together with their side groups.
    2. Remove the two bonds that cross the brackets.
    3. Change the C-C\text{C-C}C-C single bond between the two carbons into a C=C\text{C=C}C=C double bond.
Example

Poly(propene) from propene

  • Propene, CH2=CHCH3\text{CH}_2\text{=CHCH}_3CH2​=CHCH3​, has a CH3\text{CH}_3CH3​ group on one carbon of its double bond.
  • Opening the double bond and keeping each group on its own carbon gives the repeat unit of poly(propene).
n HCH3∣∣C=C∣∣HH→[HCH3∣∣−C−C−∣∣HH]n n\ \begin{array}{ccc} \text{H} & & \text{CH}_3 \\ | & & | \\ \text{C} & = & \text{C} \\ | & & | \\ \text{H} & & \text{H} \end{array} \rightarrow \left[ \begin{array}{ccccc} & \text{H} & & \text{CH}_3 & \\ & | & & | & \\ - & \text{C} & - & \text{C} & - \\ & | & & | & \\ & \text{H} & & \text{H} & \end{array} \right]_n n H∣C∣H​=​CH3​∣C∣H​→​−​H∣C∣H​−​CH3​∣C∣H​−​​n​
  • In structural formulae the same equation is written as shown below.
n CH2=CHCH3→[−CH2−CH(CH3)−]n n\,\text{CH}_2\text{=CHCH}_3 \rightarrow \left[-\text{CH}_2-\text{CH(CH}_3\text{)}-\right]_n nCH2​=CHCH3​→[−CH2​−CH(CH3​)−]n​
  • A longer section of the chain shows a CH3\text{CH}_3CH3​ group on every second carbon atom, so the repeat unit is any two neighbouring backbone carbons.
HCH3HCH3HCH3∣∣∣∣∣∣−C−C−C−C−C−C−∣∣∣∣∣∣HHHHHH \begin{array}{ccccccccccccc} & \text{H} & & \text{CH}_3 & & \text{H} & & \text{CH}_3 & & \text{H} & & \text{CH}_3 & \\ & | & & | & & | & & | & & | & & | & \\ - & \text{C} & - & \text{C} & - & \text{C} & - & \text{C} & - & \text{C} & - & \text{C} & - \\ & | & & | & & | & & | & & | & & | & \\ & \text{H} & & \text{H} & & \text{H} & & \text{H} & & \text{H} & & \text{H} & \end{array} −​H∣C∣H​−​CH3​∣C∣H​−​H∣C∣H​−​CH3​∣C∣H​−​H∣C∣H​−​CH3​∣C∣H​−​
Example

Chloroethene from its polymer

  • The repeat unit of poly(chloroethene) has a two-carbon backbone with three hydrogen atoms and one chlorine atom.
[HCl∣∣−C−C−∣∣HH]n \left[ \begin{array}{ccccc} & \text{H} & & \text{Cl} & \\ & | & & | & \\ - & \text{C} & - & \text{C} & - \\ & | & & | & \\ & \text{H} & & \text{H} & \end{array} \right]_n ​−​H∣C∣H​−​Cl∣C∣H​−​​n​
  • Removing the two outer bonds and turning the C-C\text{C-C}C-C bond into C=C\text{C=C}C=C gives the monomer, chloroethene.
HCl∣∣C=C∣∣HH \begin{array}{ccc} \text{H} & & \text{Cl} \\ | & & | \\ \text{C} & = & \text{C} \\ | & & | \\ \text{H} & & \text{H} \end{array} H∣C∣H​=​Cl∣C∣H​
  • The monomer has the formula CH2=CHCl\text{CH}_2\text{=CHCl}CH2​=CHCl, which contains the same atoms as the repeat unit.

Condensation polymerisation

Definition

Condensation polymerisation

A reaction in which many monomers, each with two functional groups, join together to form a polymer and a small molecule such as water.

  1. In condensation polymerisation, each monomer has two functional groups, one at each end of the molecule.

  2. A functional group on one monomer reacts with a functional group on another monomer to form a linkage.

  3. Each time a linkage forms, a small molecule is given off, and for the polymers you need this small molecule is water.

  4. Because each monomer has a reactive group at both ends, the chain can keep growing in both directions.

  5. The usual monomers come in three types.

    1. A dicarboxylic acid has a carboxylic acid group, -COOH\text{-COOH}-COOH, at each end.
    2. A diamine has an amine group, -NH2\text{-NH}_2-NH2​, at each end.
    3. A diol has an alcohol group, -OH\text{-OH}-OH, at each end.

  6. The carbon chain between the two functional groups does not take part in the reaction, so it is often drawn as a block (a box).

    1. Here, an empty box A\boxed{\phantom{A}}A​ stands for the carbon chain of the dicarboxylic acid.
    2. A shaded box ■\boxed{\blacksquare}■​ stands for the carbon chain of the diamine or the diol.

  7. The monomers are drawn with block diagrams like this.

    HOOC−A−COOHH2N−■−NH2HO−■−OH \text{HOOC}-\boxed{\phantom{A}}-\text{COOH} \qquad \text{H}_2\text{N}-\boxed{\blacksquare}-\text{NH}_2 \qquad \text{HO}-\boxed{\blacksquare}-\text{OH} HOOC−A​−COOHH2​N−■​−NH2​HO−■​−OH

Polyamides

Definition

Polyamide

A condensation polymer in which the monomer units are joined together by amide linkages.

Definition

Amide linkage

The -CONH- group, formed when a carboxylic acid group reacts with an amine group, that joins neighbouring units in a polyamide.

  1. A polyamide forms when a dicarboxylic acid reacts with a diamine.

  2. The -COOH\text{-COOH}-COOH group of the acid reacts with the -NH2\text{-NH}_2-NH2​ group of the amine.

    1. The -OH\text{-OH}-OH from the acid group and one H\text{H}H from the amine group leave together as a molecule of water.
    2. The carbon and nitrogen atoms then bond to each other, forming an amide linkage.

  3. The reaction at one linkage is shown below.

    −COOH+H2N−→−CONH−+H2O -\text{COOH} + \text{H}_2\text{N}- \rightarrow -\text{CONH}- + \text{H}_2\text{O} −COOH+H2​N−→−CONH−+H2​O

  4. Drawn out fully, the amide linkage contains a C=O\text{C=O}C=O group bonded to an N-H\text{N-H}N-H group.

    OH∥∣−C−N− \begin{array}{ccccc} & \text{O} & & \text{H} & \\ & \| & & | & \\ - & \text{C} & - & \text{N} & - \end{array} −​O∥C​−​H∣N​−​

  5. Using block diagrams, the equation for making a polyamide is shown below.

    n HOOC−A−COOH+n H2N−■−NH2→[−CO−A−CO−NH−■−NH−]n+2n H2O n\,\text{HOOC}-\boxed{\phantom{A}}-\text{COOH} + n\,\text{H}_2\text{N}-\boxed{\blacksquare}-\text{NH}_2 \rightarrow \left[-\text{CO}-\boxed{\phantom{A}}-\text{CO}-\text{NH}-\boxed{\blacksquare}-\text{NH}-\right]_n + 2n\,\text{H}_2\text{O} nHOOC−A​−COOH+nH2​N−■​−NH2​→[−CO−A​−CO−NH−■​−NH−]n​+2nH2​O

  6. Each repeat unit contains one acid block and one amine block, and two water molecules are lost for every repeat unit formed.

Polyesters

Definition

Polyester

A condensation polymer in which the monomer units are joined together by ester linkages.

Definition

Ester linkage

The -COO- group, formed when a carboxylic acid group reacts with an alcohol group, that joins neighbouring units in a polyester.

  1. A polyester forms when a dicarboxylic acid reacts with a diol.

  2. The -COOH\text{-COOH}-COOH group of the acid reacts with the -OH\text{-OH}-OH group of the diol.

    1. The -OH\text{-OH}-OH from the acid group and the H\text{H}H from the alcohol group leave together as water.
    2. The carbon atom of the acid then bonds to the oxygen atom of the diol, forming an ester linkage.

  3. The reaction at one linkage is shown below.

    −COOH+HO−→−COO−+H2O -\text{COOH} + \text{HO}- \rightarrow -\text{COO}- + \text{H}_2\text{O} −COOH+HO−→−COO−+H2​O

  4. Drawn out fully, the ester linkage contains a C=O\text{C=O}C=O group with a single-bonded O\text{O}O next to it.

    O∥−C−O− \begin{array}{ccccc} & \text{O} & & & \\ & \| & & & \\ - & \text{C} & - & \text{O} & - \end{array} −​O∥C​−​O​−​

  5. Using block diagrams, the equation for making a polyester is shown below.

    n HOOC−A−COOH+n HO−■−OH→[−CO−A−CO−O−■−O−]n+2n H2O n\,\text{HOOC}-\boxed{\phantom{A}}-\text{COOH} + n\,\text{HO}-\boxed{\blacksquare}-\text{OH} \rightarrow \left[-\text{CO}-\boxed{\phantom{A}}-\text{CO}-\text{O}-\boxed{\blacksquare}-\text{O}-\right]_n + 2n\,\text{H}_2\text{O} nHOOC−A​−COOH+nHO−■​−OH→[−CO−A​−CO−O−■​−O−]n​+2nH2​O

  6. Polyamides and polyesters are made in the same way, and the only difference is the linkage: -CONH-\text{-CONH-}-CONH- in a polyamide and -COO-\text{-COO-}-COO- in a polyester.

Example

A polyester from butanedioic acid and ethane-1,2-diol

  • Butanedioic acid, HOOC-CH2CH2-COOH\text{HOOC-CH}_2\text{CH}_2\text{-COOH}HOOC-CH2​CH2​-COOH, is a dicarboxylic acid.
  • Ethane-1,2-diol, HO-CH2CH2-OH\text{HO-CH}_2\text{CH}_2\text{-OH}HO-CH2​CH2​-OH, is a diol.
  • Each -COOH\text{-COOH}-COOH group loses -OH\text{-OH}-OH and each -OH\text{-OH}-OH group of the diol loses H\text{H}H, so water forms at every linkage.
  • The acid section and the diol section then join through ester linkages.
n HOOC−CH2CH2−COOH+n HO−CH2CH2−OH→[−CO−CH2CH2−CO−O−CH2CH2−O−]n+2n H2O n\,\text{HOOC}-\text{CH}_2\text{CH}_2-\text{COOH} + n\,\text{HO}-\text{CH}_2\text{CH}_2-\text{OH} \rightarrow \left[-\text{CO}-\text{CH}_2\text{CH}_2-\text{CO}-\text{O}-\text{CH}_2\text{CH}_2-\text{O}-\right]_n + 2n\,\text{H}_2\text{O} nHOOC−CH2​CH2​−COOH+nHO−CH2​CH2​−OH→[−CO−CH2​CH2​−CO−O−CH2​CH2​−O−]n​+2nH2​O
  • The repeat unit contains one acid section and one diol section, with an ester linkage between them and another at the end where the next unit joins.

Working back to the monomers

  1. To find the monomers of a condensation polymer, first find the linkages in the repeat unit.
    1. A -CONH-\text{-CONH-}-CONH- linkage means the polymer is a polyamide, so the monomers are a dicarboxylic acid and a diamine.
    2. A -COO-\text{-COO-}-COO- linkage means the polymer is a polyester, so the monomers are a dicarboxylic acid and a diol.
  2. Break each linkage between the C\text{C}C of the C=O\text{C=O}C=O group and the N\text{N}N or O\text{O}O next to it.
  3. Put back the atoms that were lost as water.
    1. Add -OH\text{-OH}-OH to each C=O\text{C=O}C=O end, which turns it back into -COOH\text{-COOH}-COOH.
    2. Add H\text{H}H to each -NH-\text{-NH-}-NH- end to make -NH2\text{-NH}_2-NH2​, or to each -O-\text{-O-}-O- end to make -OH\text{-OH}-OH.
  4. Check that each monomer now has the same functional group at both ends.
Example

Monomers of a polyamide

  • The repeat unit below contains -CONH-\text{-CONH-}-CONH- linkages, so the polymer is a polyamide.
[−NH−CH2CH2−NH−CO−CH2−CO−]n \left[-\text{NH}-\text{CH}_2\text{CH}_2-\text{NH}-\text{CO}-\text{CH}_2-\text{CO}-\right]_n [−NH−CH2​CH2​−NH−CO−CH2​−CO−]n​
  • Breaking the bonds between each C=O\text{C=O}C=O carbon and its neighbouring N\text{N}N separates a section with two NH\text{NH}NH ends from a section with two CO\text{CO}CO ends.
  • Adding H\text{H}H to each NH\text{NH}NH end gives the diamine.
H2N−CH2CH2−NH2 \text{H}_2\text{N}-\text{CH}_2\text{CH}_2-\text{NH}_2 H2​N−CH2​CH2​−NH2​
  • Adding OH\text{OH}OH to each CO\text{CO}CO end gives the dicarboxylic acid.
HOOC−CH2−COOH \text{HOOC}-\text{CH}_2-\text{COOH} HOOC−CH2​−COOH
  • Both monomers have the same functional group at each end, which confirms the answer.

Addition versus condensation

  1. Monomers: addition polymerisation uses monomers with a C=C\text{C=C}C=C double bond, while condensation polymerisation uses monomers with two functional groups.
  2. Products: addition polymerisation forms the polymer as the only product, while condensation polymerisation also forms a small molecule, such as water.
  3. Number of monomer types: an addition polymer is usually made from one type of monomer, while the condensation polymers here are made from two different monomers.
  4. Repeat unit: an addition polymer repeat unit has the same atoms as its monomer, while a condensation polymer repeat unit has fewer atoms than the monomers that made it, because water has been lost.
  5. Linkage: an addition polymer has a backbone of C-C\text{C-C}C-C bonds, while a condensation polymer contains amide or ester linkages.
Common Mistake
  • Do not leave a C=C\text{C=C}C=C double bond inside the brackets of an addition polymer, because the double bond opens during the reaction.
  • Do not put a side group such as CH3\text{CH}_3CH3​ into the main chain, because only the two carbons of the double bond form the backbone.
  • Do not forget the water when you write a condensation polymerisation equation, because a condensation reaction always gives a small molecule as well as the polymer.
  • Do not show an -OH\text{-OH}-OH or -NH2\text{-NH}_2-NH2​ group inside the repeat unit of a condensation polymer, because these groups have been used up in the linkages.
  • Do not draw an ester linkage with two oxygen atoms in a row, -O-O-\text{-O-O-}-O-O-, because the correct linkage is -CO-O-\text{-CO-O-}-CO-O-.
Exam technique

Drawing polymer structures

  • Draw the linkage in full, showing the C=O\text{C=O}C=O and the N-H\text{N-H}N-H in an amide linkage, because a squashed formula in the middle of a chain is easy to misread.
  • Mark schemes often give one mark for the correct linkage and another for the continuation bonds, so make sure both ends of the repeat unit have a bond crossing the bracket.
  • When asked for one difference between addition and condensation polymerisation, the most reliable answer is that condensation polymerisation also produces a small molecule such as water.
  • If you are given a long chain, count along to find where the pattern starts again, then draw only one repeat unit.
Self review
  • What is a repeat unit?
  • Draw the repeat unit of the addition polymer made from propene.
  • Which two types of monomer react to form a polyamide?
  • What is the formula of an ester linkage?
  • Give two differences between addition and condensation polymerisation.
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11.8.3 Addition and condensation polymers Revision Guide

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Revision notes for CIE IGCSE Chemistry 11.8.3 Addition and condensation polymers: explanations and worked examples.

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